To be excruciatingly precise, the best deal is the (1, 3, or 6 month) MONTHLY memberships. (The term "membership" is ambiguous.) As long as you're a monthly member, there should never be a day that you don't get a brand new card that you don't already own. If for some reason you ever get a duplicate, they'll make it right. If you're already a monthly member, the next best deal and the best way to grow your collection (faster than one card per day) is with the booster packs.
I do hope they'll give us fair warning before discontinuing the booster pack deals, or better still, just tell us booster packs will be here forever. Inquiring minds want to know.